On Thu, 13 Dec 2018, Paul E. McKenney wrote: > > > A good next step would be to automatically generate random tests along > > > with an automatically generated prediction, like I did for RCU a few > > > years back. I should be able to generalize my time-based cheat for RCU to > > > also cover SRCU, though sys_membarrier() will require a bit more thought. > > > (The time-based cheat was to have fixed duration RCU grace periods and > > > RCU read-side critical sections, with the grace period duration being > > > slightly longer than that of the critical sections. The number of > > > processes is of course limited by the chosen durations, but that limit > > > can easily be made insanely large.) > > > > Imagine that each sys_membarrier call takes a fixed duration and each > > other instruction takes slightly less (the idea being that each > > instruction is a critical section). Instructions can be reordered > > (although not across a sys_membarrier call), but no matter how the > > reordering is done, the result is disallowed. This turns out not to be right. Instead, imagine that each sys_membarrier call takes a fixed duration, T. Other instructions can take arbitrary amounts of time and can be reordered abitrarily, with two restrictions: Instructions cannot be reordered past a sys_membarrier call; If instructions A and B are reordered then the time duration from B to A must be less than T. If you prefer, you can replace the second restriction with something a little more liberal: If A and B are reordered and A ends up executing after a sys_membarrier call (on any CPU) then B cannot execute before that sys_membarrier call. Of course, this form is a consequence of the more restrictive form. > It gets a bit trickier with interleavings of different combinations > of RCU, SRCU, and sys_membarrier(). Yes, your cat code very elegantly > sorts this out, but my goal is to be able to explain a given example > to someone. I don't think you're going to be able to fit different combinations of RCU, SRCU, and sys_membarrier into this picture. How would you allow tests with incorrect interleaving, such as GP - memb - RSCS - nothing, while forbidding similar tests with correct interleaving? Alan
